Hybrid and Electric Vehicle Electrical Systems and High-Voltage Safety
What the exam tests
High-voltage system layout, safety procedures, isolation testing and 12 V interaction.
Core
- Traction batteries commonly operate at 300–400 V DC, with some systems at 800 V. Cables are orange. The system includes the battery pack, BMS, service disconnect, inverter, motor-generator, DC-DC converter, charger, electric A/C compressor and PTC heater.
- High voltage can kill. Use Class 0 insulating gloves rated for 1000 V, insulated tools, lockout/tagout, and the one-hand rule. Remove jewellery.
- Disconnect the service plug, wait the manufacturer's specified time (often 5–10 minutes) for capacitors to discharge, then verify absence of voltage with a CAT III meter on the correct range.
- Do not assume an orange cable is dead. Interlocks detect cover removal and may disable the system but do not remove stored energy.
- Isolation faults are found with an insulation tester; do not use an ordinary ohmmeter. The DC-DC converter charges the 12 V battery, so a no-start can be a 12 V fault.
Worked example
A hybrid shows a high-voltage isolation DTC. Park and isolate the vehicle, disconnect the service plug, wait, and verify 0 V. Use an insulation tester between the HV bus and chassis; a low reading confirms leakage. Disconnect sections one at a time - motor, inverter, compressor, heater - and retest to locate the faulty component. Never use your body or a normal multimeter as the test.
Traps
Treating orange cables as low voltage; using a CAT I meter on HV; skipping the discharge wait; testing insulation with a standard ohmmeter; wearing damaged gloves; forgetting that the 12 V system still powers modules.